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Abstract:

An electrode for a lithium secondary battery includes a silicon-based
alloy, and has a surface roughness of about 1 to about 10 μm and a
surface roughness deviation of 5 μm or less. A method of manufacturing
the electrode includes mixing an electrode composition, milling the
composition, coating the milled composition on a current collector, and
drying the milled composition. A lithium secondary battery includes the
electrode.

Claims:

1. An electrode for a lithium secondary battery, comprising an electrode
active material comprising a silicon-based alloy, the electrode having a
surface roughness of about 1 to about 10 μm and a surface roughness
deviation of about 5 μm or less.

2. The electrode of claim 1, wherein an amount of silicon in the
silicon-based alloy is about 60 atom % to about 72 atom %.

4. The electrode of claim 3, wherein an amount of the active silicon is
about 40 atom % to about 80 atom % based on a total amount of the active
silicon and the inactive silicon in the silicon-based alloy.

5. The electrode of claim 1, wherein the surface roughness deviation is
about 2 μm or less.

6. The electrode of claim 1, wherein the electrode has a mixed density of
about 0.80 to about 0.90 g/cc.

7. The electrode of claim 1, wherein the silicon-based alloy comprises
silicon and at least one additional metal selected from the group
consisting of Al, Ni, Fe, Mn, and Ti.

8. The electrode of claim 1, wherein the silicon-based alloy comprises an
alloy represented by Si-M-A, wherein: M is Al, Ti or Fe; A is Ni, Fe or
Mn; and M and A are different from each other.

9. The electrode of claim 8, wherein an amount of Si in the Si-M-A alloy
is about 60 atom % to about 72 atom %, an amount of M in the Si-M-A alloy
is about 7 atom % to about 20 atom %, and an amount of A in the Si-M-A
alloy is about 15 atom % to about 20 atom %.

10. The electrode of claim 1, wherein the silicon-based alloy comprises
an alloy selected from the group consisting of
Si68Al8Ni24, Si60Ti20Ni20,
Si70Fe15Mn15, Si70Al15Fe15,
Si70Al15Mn15, Si70Ti15Fe15,
Si65Ti.sub.17.5Ni.sub.17.5, and Si68Ti16Ni.sub.16.

11. The electrode of claim 1, wherein the electrode active material is a
negative electrode active material, and the electrode is a negative
electrode.

12. The electrode of claim 11, wherein the electrode active material
further comprises a second material selected from the group consisting of
carbonaceous materials, lithium metal, lithium alloys, silicon-oxide
based materials, and combinations thereof.

13. The electrode of claim 12, wherein an amount of the second material
is about 1 to about 99 parts by weight based on 100 parts by weight of
the silicon-based alloy and the second material.

14. A method of manufacturing an electrode for a lithium secondary
battery, the method comprising: mixing a conductive material, a binder, a
solvent, and an electrode active material comprising a silicon-based
alloy to make an electrode composition; milling the electrode composition
to form a milled electrode composition; coating the milled electrode
composition on a current collector; and drying the milled electrode
composition to form the electrode, the electrode having a surface
roughness of about 1 μm to about 10 μm and a surface roughness
deviation of about 5 μm or less.

15. The method of claim 14, wherein the electrode active material in the
milled electrode composition has an average particle diameter of about 10
μm or smaller.

16. The method of claim 14, wherein the solvent is present in the
electrode composition in an amount sufficient to yield a solids content
of about 30 wt % to about 50 wt %.

17. The method of claim 14, wherein the mixing comprises: first loading
the solvent into a milling machine to form a liquid condition; after
loading the solvent into the milling machine, adding the electrode active
material, the conductive material and the binder to the milling machine.

18. The method of claim 14, wherein the milling is carried out in a bead
mill including beads having an average diameter of about 0.5 to about 2
mm, and a speed of about 1000 rpm to about 2000 rpm.

19. The method of claim 14, wherein the drying is performed at a
temperature of about 100.degree. C. to about 150.degree. C.

20. A lithium secondary battery, comprising: a first electrode comprising
the electrode of claim 1; a second electrode; and an electrolyte.

Description:

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to and the benefit of U.S.
Provisional Application No. 61/601,420, filed on Feb. 21, 2012 in the
U.S. Patent and Trademark Office, the entire content of which is
incorporated herein by reference

BACKGROUND

[0002] 1. Technical Field

[0003] The present invention relates to an electrode for a lithium
secondary battery, a method of manufacturing the electrode, and a lithium
secondary battery including the electrode.

[0004] 2. Description of Related Art

[0005] Lithium secondary batteries that are attractive for use as power
sources for small and portable electronic devices use organic
electrolytic solutions. Due to the use of organic electrolytic solutions,
lithium secondary batteries have a discharge voltage that is two or more
times greater than that of a typical battery using an alkali aqueous
solution, and thus lithium secondary batteries have high energy
densities.

[0006] The positive and negative electrodes of a lithium secondary battery
include materials that enable intercalation and deintercalation of
lithium ions, and the space between the positive and negative electrodes
is filled with an organic electrolytic solution or a polymer electrolytic
solution. Electric energy is generated by oxidation and reduction
reactions occurring when lithium ions are intercalated into or
deintercalated from the positive electrode and the negative electrode.

[0007] Although such lithium secondary batteries have good battery
properties including high electromotive force and high energy density,
developments in industry require batteries having longer lifespan
characteristics.

SUMMARY

[0008] According to embodiments of the present invention, an electrode for
a lithium secondary battery has improved lifespan characteristics. Other
embodiments are directed to a method of manufacturing the electrode.

[0009] Another embodiment of the present invention provides a lithium
secondary battery including the electrode.

[0010] According to one or more embodiments of the present invention, an
electrode for a lithium secondary battery includes a silicon-based alloy
and has a surface roughness of about 1 to about 10 μm, and a surface
roughness deviation of 5 μm or less.

[0011] According to one or more embodiments of the present invention, a
method of manufacturing an electrode includes preparing a composition for
forming an electrode active material layer by adding an active material,
a conductive material, and a binder to a solvent, followed by wet mixing
and milling; and coating and drying the composition on a current
collector.

[0012] According to one or more embodiments of the present invention, a
lithium secondary battery includes the electrode described above.

[0013] According to embodiments of the present invention, electrodes for a
lithium secondary battery are efficiently impregnated with an
electrolytic solution and thus, lithium secondary batteries including
such electrodes have improved efficiency, capacity retention rates and
lifespan characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] FIG. 1 is a diagram explaining the electrolytic solution
impregnation property of a negative electrode according to an embodiment
of the present invention.

[0015]FIG. 2 is a cross-sectional perspective view of a lithium secondary
battery according to an embodiment of the present invention.

[0016] FIG. 3 is an scanning electron microscope (SEM) image of a negative
electrode manufactured using the composition for forming a negative
active material layer prepared according to Example 1.

[0017] FIG. 4 is a SEM image of a negative electrode manufactured using
the composition for forming a negative active material layer prepared
according to Comparative Example 2.

[0018]FIG. 5 is a graph comparing the expansion ratio with respect to
volumetric capacity of the electrodes manufactured according to Example 1
and Comparative

[0019] Examples 1-4.

[0020]FIG. 6 is a graph comparing the cycle characteristics of the coin
cells manufactured according to Manufacture Example 1 and Comparative
Manufacture Examples 1, 2, 3, and 5.

DETAILED DESCRIPTION

[0021] An electrode for a lithium secondary battery, according to an
embodiment of the present invention, includes a silicon-based alloy, and
has a surface roughness of about 1 to about 10 μm, and a surface
roughness deviation of 5 μm or less.

[0022] Silicon-based alloys can be used as electrode active materials, but
to obtain high capacity, silicon-based alloys having high silicon content
are used. When electrodes are manufactured using such silicon-based
alloys containing high silicon content, the crystal size of the
silicon-based alloy increases and thus, the formed electrode may have a
surface roughness of greater than 10 μm. Thus, the electrolytic
solution impregnation property with respect to the electrode is reduced,
causing a solid electrolyte interface (SEI) to be continuously formed on
the surface of the electrode when the silicon volumetrically expands.
Consequently, a lithium secondary battery employing the electrode may
have decreased initial efficiency, and a substantially decreased capacity
retention rate (C.R.R). As a result, the lithium secondary battery may
have a decreased lifespan.

[0023] In contrast, the electrodes according to embodiments of the present
invention have an appropriately controlled silicon content and mixed
ratio of active silicon to inactive silicon in the silicon-based alloy.
In addition, according to some embodiments of the present invention,
optimum mixing and milling conditions of the silicon-based alloy, binder,
and conductive material yield a formed electrode having a surface
roughness of 1 to 10 μm and a surface roughness deviation of 5 μm
or less.

[0024] The amount of silicon in the silicon-based alloy may be about 60 to
about 72 atom %. If the amount of the silicon in the silicon-based alloy
is within this range, electrodes using the silicon-based alloy may have
good surface roughness and roughness deviation characteristics.

[0025] The silicon of the silicon-based alloy may contain inactive silicon
and active silicon, which are mixed. The active silicon may directly
affect the capacity of the silicon-based alloy, and the inactive silicon
may have an inactive matrix structure and may suppress volumetric
expansion of the silicon-based alloy.

[0026] The amount of active silicon may be about 40 to about 80 atom %
based on 100 atom % of the total amount of the active silicon and the
inactive silicon in the silicon-based alloy. If the amount of active
silicon is within this range, when an electrode including the active
silicon is charged and discharged, volumetric expansion of the
silicon-based alloy may be efficiently suppressed and the electrode may
have good capacity characteristics.

[0027] The silicon-based alloy has silicon particles dispersed within a
silicon alloy-based matrix. Due to the structure and composition of the
silicon-based alloy, when the silicon particles expand during charging
and discharging, the silicon alloy-based matrix surrounding the silicon
particles may efficiently control the volumetric change in the silicon.
Accordingly, when the silicon-based alloy is used as a negative active
material, the expansion ratio of an electrode using the silicon-based
alloy may be reduced during charging and discharging.

[0028] As described above, due to the decrease in the expansion ratio of
an electrode during charging and discharging, problems resulting from the
expansion of the electrode (that is, decreased lifespan characteristics
resulting from an increased irreversible capacity of lithium due to the
formation of an additional SEI film occurring when the silicon
alloy-based matrix is destroyed) may be substantially prevented.

[0029] Also, the formed electrode may have the surface roughness and
surface roughness deviation characteristics described above, and thus,
the initial electrolytic solution impregnation property may be improved,
and deviations resulting from a local overpotential may be reduced. Thus,
lifespan may be improved, and the battery manufacture yield may be
increased.

[0030] An electrode for a lithium secondary battery, according to an
embodiment of the present invention, is described below with reference to
FIG. 1.

[0031] Referring to FIG. 1, A depicts an electrode having a substantially
uniform surface roughness, and B depicts an electrode with a surface
roughness deviation that is greater than that of the electrode of A.

[0032] Referring to FIG. 1, when the roughness of the electrode is
uniformly controlled, as illustrated in A, the specific surface area of
the electrode surface is increased, and thus, the impregnation
characteristics of the electrolytic solution are improved. When the
impregnation characteristics of the electrolytic solution are improved,
the silicon alloy matrix substantially suppresses the formation of a SEI
film, and thus, expansion of the negative electrode including the silicon
alloy-based material after charging and discharging may be effectively
suppressed.

[0033] When expansion after charging and discharging is effectively
suppressed in the electrodes, as described above, lithium batteries
formed using the electrodes may have improved initial efficiency,
capacity retention rates, and lifespan characteristics.

[0034] The electrode may have a roughness standard deviation of, for
example, 2 μm or less, for example, about 0.1 to about 2 μm.

[0035] The term "surface roughness," as used herein, refers to an
evaluation of the levels or amount of pin holes or protrusions, mixed
impurities, cracks, strip patterns, lumps, etc., and refers to a surface
roughness arithmetical mean value of the electrode.

[0036] According to embodiments of the present invention, the electrode
may have a mixed density of about 0.80 to about 0.90 g/cc.

[0037] The "mixed density" is calculated by dividing the weight of the
components of the electrode (other than the current collector, i.e.,
active material, conductive material, binder, etc.) by the volume of the
electrode.

[0038] When the mixed density of the electrode is within the above range,
the surface roughness characteristics of the electrode are controlled
within an appropriate range. Thus, when an electrode having such surface
roughness characteristics is employed, the lifespan of the formed lithium
secondary battery may be improved.

[0039] The silicon-based alloy may include silicon, and one or more metals
selected from aluminum (Al), nickel (Ni), iron (Fe), manganese (Mn), and
titanium (Ti).

[0040] The silicon-based alloy may be represented by, for example,
silicon-M-A. In this formula, M and A are different from each other, and
M may be, for example, aluminum (Al), titanium (Ti), or iron (Fe), and A
may be, for example, nickel (Ni), iron (Fe), or manganese (Mn).

[0041] The amount of silicon in the silicon-M-A alloy may be about 60 to
about 72 atom %.

[0042] The amount of M in the silicon-M-A alloy may be about 7 to about 20
atom %, and an amount of A in the silicon-M-A alloy may be about 15 to
about 20 atom %.

[0044] Some embodiments of the present invention are directed to a method
of manufacturing an electrode for a lithium secondary battery. The
electrode contains a silicon-based alloy.

[0045] The conditions used for mixing the silicon-based alloy, the
conductive material, and the binder may significantly affect the surface
roughness characteristics of the finally obtained electrode.

[0046] According to an embodiment of the present invention, a composition
for forming an electrode active material layer may be prepared as
described below. The composition includes a silicon-based alloy as an
electrode active material, a conductive material, and a binder.

[0047] To mix the silicon-based alloy, conductive material, and binder of
the composition for forming the electrode active material layer, a
solvent is loaded into a milling machine to prepare a liquid condition,
and then the active material, conductive material, and binder are added
thereto, followed by mixing. The adding sequence of the active material,
the conductive material, and the binder is not particularly limited, and
for example, the active material may be added first followed by the
conductive material and the binder. By doing this, pulverization of the
active material into too small particles may be substantially suppressed.

[0048] The mixing of the active material, the conductive material, and the
binder may be performed by wet mixing and milling in a liquid condition.
Through the wet mixing and milling, oxidation of the active material, the
conductive material, and the binder may be prevented, and also, the
particle diameter of the active material may be controlled to be about 10
μm or less, for example, about 1 μm to about 7 μm. If the
particle diameter of the active material is controlled within this range,
problems resulting from the volumetric expansion of silicon-based
particles during charging and discharging may be substantially prevented,
and also, the surface characteristics of the active material may be
optimized so that the surface of the active material is uniformly wetted
with the electrolytic solution.

[0049] When the particle diameter of the active material is controlled to
be 10 μm or less and the surface characteristics of the active
material are controlled as described above, the formed electrode may have
a surface roughness of about 1 μm to about 10 μm and a surface
roughness deviation of about 5 μm or less. Thus, pulverization of the
electrode active material into too small particles and damage to the
electrode active material may be effectively suppressed during pressing
(which is performed during manufacture of the electrode) and/or during
charging and discharging of the lithium secondary battery employing the
electrode.

[0050] The solvent may be N-methylpyrrolidone (NMP), pure water, or the
like.

[0051] The amount of the solvent may be such that the solids content of
the composition is about 30 to about 50 wt %. If the amount of the
solvent is within this range, the particle diameter of the active
material may be controlled to be 10 μm or less, and the components of
the composition may have good dispersion properties, enabling the active
material layer to be easily formed.

[0052] As a milling machine for wet mixing and milling, for example, a
bead mill may be used. When a bead mill is used, the silicon-based alloy
as an active material, the conductive material, and the binder are milled
into appropriate particle sizes and are uniformly dispersed. Thus, the
formed electrode may have good surface roughness characteristics.

[0053] If the particle diameter of the active material is controlled to be
10 μm or less as described above, pulverization of the silicon-based
alloy active material into too small particles during charging and
discharging and/or during pressing of the electrode may be substantially
prevented.

[0054] The wet mixing and milling is performed using beads, and an average
particle size of the beads may be, for example, about 0.5 to about 2 mm,
and the revolutions per minute (rpm) of the milling machine may be, for
example, about 1000 to about 2000 rpm. When the average diameter of the
beads and the revolutions per minute of the mill are within these ranges,
the prepared composition may have good dispersibility and pulverization
of the active material into too small particles may be substantially
prevented.

[0055] Non-limiting examples of materials for the beads include zirconia
beads and alumina beads.

[0056] The resultant that has been subjected to the wet-mixing and milling
is coated on an electrode current collector, and then dried and pressed
to complete the manufacture of an electrode.

[0057] The drying may be performed at a temperature of about 100 to about
150° C., for example, about 100 to about 110° C.

[0058] The electrode active material may be, for example, a negative
active material, and the electrode may be, for example, a negative
electrode.

[0059] The negative active material may include, the silicon-based alloy
described above, and may further include other negative active materials
(i.e., a second active material), such as those typically used in lithium
secondary batteries.

[0060] Non-limiting examples of suitable second negative active materials
include materials capable of intercalating or deintercalating lithium
ions, such as graphite, carbonaceous materials, such as carbon, lithium
metal and alloys thereof, silicon oxide-based materials, mixtures
thereof, and the like.

[0061] According to an embodiment of the present invention, the negative
active material may include the silicon-based alloy and a carbonaceous
material. The carbonaceous material may be graphite, or pitch (which is
amorphous carbon).

[0062] When the carbonaceous material is used together with the
silicon-based alloy as described above, oxidation of the silicon-based
alloy is suppressed and a SEI film may be stably and effectively formed.
Also, electric conductivity may be increased to further improve the
charging and discharging characteristics of the lithium battery.

[0063] When the carbonaceous material is used, the carbonaceous material
may be coated on a surface of the silicon-based alloy.

[0064] The amount of the second negative active material may be about 1 to
about 99 parts by weight based on 100 parts by weight of the total amount
of the silicon-based alloy and the second negative active material.

[0065] If the negative active material includes the silicon-based alloy as
a major component, the amount of the silicon-based alloy may be, for
example, about 95 to about 99 parts by weight based on 100 parts by
weight of the total amount of the second negative active material and the
silicon-based alloy. When graphite or pitch is used as the second
negative active material, the graphite or pitch may be coated on the
surface of the silicon-based alloy.

[0066] If the negative active material includes the silicon-based alloy as
a minor component, the amount of the silicon-based alloy may be, for
example, about 1 to about 5 parts by weight based on 100 parts by weight
of the total amount of the second negative active material and the
silicon-based alloy. When graphite or pitch is used as the second
negative active material, the graphite or pitch may function as a buffer
for the silicon-based alloy, and thus the lifespan of the formed
electrode may be further increased.

[0067] The binder is used in an amount of about 1 to about 10 parts by
weight based on 100 parts by weight (the total weight) of the negative
active material. Non-limiting examples of the binder include
polyvinylidenefluoride, polyvinylalcohol, carboxymethylcellulose (CMC),
starch, hydroxypropylcellulose, regenerated cellulose,
polyvinylpyrrolidone, polyamide imide, polyethylene, polypropylene,
ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene
butadiene rubber, fluoride rubber, and vinylidene fluoride copolymers
having one or more functional groups selected from carboxyl groups, epoxy
groups, hydroxyl groups, and carbonyl groups.

[0068] When the amount of the binder is within the above range, the
binding force of the electrode active material with respect to the
current collector is further enhanced, and thus, the electrode and
resulting battery may have improved lifespan and stability.

[0069] The conductive material may be used in an amount of about 1 to
about 10 parts by weight based on 100 parts by weight (the total weight)
of the negative active material. When the amount of the conductive
material is within this range, the formed electrode may have good
conductivity characteristics.

[0070] The conductive material is not particularly limited and may be any
one of various materials so long as it is conductive and does not cause
any chemical change in the corresponding battery. Non-limiting examples
thereof include graphite, such as natural or artificial graphite;
carbonaceous materials, such as carbon black, acetylene black, Ketjen
black, channel black, furnace black, lamp black, and thermal black;
conductive fibers such as carbon fibers and metallic fibers; fluoride
carbon; metal powders, such as aluminum or nickel powder; conductive
whiskers such as zinc oxide and potassium titanate; conductive metal
oxides such as titanium oxide; and conductive materials, such as
polyphenylene derivatives.

[0071] The negative electrode current collector may have a thickness of,
in general, about 3 to about 500 μm. The material for forming the
negative electrode current collector is not particularly limited and may
be any one of various materials so long as it is conductive and does not
cause any chemical change in the corresponding battery.

[0072] Non-limiting examples of such a material include copper; stainless
steel; aluminum, nickel; titanium; heat treated carbon; copper and
stainless steel, each of which has a surface coated with carbon, nickel,
titanium, or silver; and aluminum-cadmium alloys. Also, like the positive
electrode current collector, the negative electrode current collector may
have an uneven surface to increase the binding force with respect to the
negative active material, and may be a film type, sheet type, foil type,
net type, porous type, foam type, a non-woven fabric type, or the like.

[0073] Hereinafter, a method of manufacturing a lithium secondary battery
using the negative electrode is described below in detail. A lithium
secondary battery according to an embodiment of the present invention
includes, for example, a positive electrode, a negative electrode, a
lithium salt-containing non-aqueous electrolyte, and a separator.

[0074] First, a positive active material, a conductive material, a binder,
and a solvent are mixed to prepare a composition for forming a positive
active material layer, and the composition is coated on a current
collector and dried to complete the manufacture of a positive electrode.

[0075] As the positive active material, any lithium transition metal oxide
that is typically used in lithium secondary batteries may be used.

[0077] The binder and the conductive material may be the same as described
above with respect tot he negative electrode, and may be used in the same
amount.

[0078] The solvent may be N-methylpyrrolidone, pure water, or the like.

[0079] The amount of the solvent may be about 1 to about 500 parts by
weight based on 100 parts by weight of the positive active material. If
the amount of the solvent is within this range, the active material layer
may be easily formed.

[0080] The positive electrode current collector may have a thickness of
about 3 to about 500 μm, and may be any one of various materials so
long as it is conductive and does not cause any chemical change in the
corresponding battery. Non-limiting examples of such a material include
stainless steel; aluminum; nickel; titanium; heat treated carbon;
aluminum and stainless steel, each of which has a surface coated with
carbon, nickel, titanium, or silver; and aluminum-cadmium alloys. The
positive electrode current collector may have an uneven surface to
increase the binding force with respect to the positive active material,
and may be a film type, a sheet type, a foil type, a net type, a porous
type, a foam type, a non-woven fabric type, or the like.

[0081] A separator is positioned between the positive electrode and the
negative electrode.

[0082] The separator may have a pore diameter of about 0.01 to about 10
μm, and a thickness of, in general, about 5 to about 300 μm. The
separator may be formed of, for example, an olefin-based polymer, such as
polypropylene, polyethylene, or the like; or a sheet or non-woven fabric
formed of glass fiber. If the electrolyte is a solid electrolyte, such as
a polymer, the solid electrolyte may also function as the separator.

[0083] The lithium salt-containing non-aqueous electrolyte may include a
non-aqueous organic solvent and a lithium salt. The non-aqueous
electrolyte may be a non-aqueous electrolytic solution, an organic solid
electrolyte, an inorganic solid electrolyte, or the like.

[0087] The lithium salt may be a material that is highly soluble in the
non-aqueous organic solvent. Non-limiting examples of the lithium salt
include LiCl, LiBr, LiI, LiClO4, LiBF4, LiB10Cl10,
LiPF6, LiCF3SO3, LiCF3CO2, LiAsF6,
LiSbF6, LiAlCl4, CH3SO3Li, CF3SO3Li, and
(CF3SO2)2NLi.

[0088]FIG. 2 is a cross-sectional perspective view of a lithium secondary
battery 30 according to an embodiment of the present invention.

[0089] Referring to FIG. 2, the lithium secondary battery 30 includes an
electrode assembly including a positive electrode 23, a negative
electrode 22, and a separator 24 disposed between the positive electrode
23 and the negative electrode 22. The electrode assembly is housed in a
battery case 25, and sealed with an encapsulation member 26. An
electrolyte (not shown) impregnates the positive electrode 23, the
negative electrode 22, and the separator 24. The negative electrode 22
may be the negative electrode according to an embodiment of the present
invention described above.

[0090] To manufacture the lithium secondary battery 30, the positive
electrode 23, the negative electrode 22, and the separator 24 are
sequentially stacked and then the stack is rolled in a jelly roll form
and then placed in the battery case 25.

[0091] One or more embodiments of the present invention will now be
described with reference to the following examples. However, the examples
are presented for illustrative purposes only, and do not limit the scope
of the present invention.

EXAMPLE 1

Manufacture of Negative Electrode

[0092] NMP was loaded into a bead mill (LabStar manufactured by NETZSCH
Company) to prepare a liquid condition. A silicon-based alloy
(Si68Al8Ni24 with a content of active silicon of about
79.4 atom %) as an active material, Ketjen black as a conductive
material, and polyamide imide (PAI) as a binder were sequentially added
thereto at a weight ratio of 88:4:8, and wet mixing and milling were
performed thereon using the bead mill at a revolutions per minute of
about 1000 rpm for about 30 minutes to prepare a composition for a
negative active material layer. The beads employed by the bead mill had a
diameter of about 0.5 mm, and NMP was used in an amount sufficient to
yield a total solids content in the composition of 48 wt %.

[0093] The composition was coated on a copper foil to form a film having a
thickness of about 14 μm, thereby forming a thin electrode. The thin
electrode was dried at a temperature of about 135° C. for 3 or
more hours, and then pressed, thereby completing the manufacture of a
negative electrode.

EXAMPLE 2

Manufacture of Negative Electrode

[0094] A negative electrode was manufactured in the same manner as Example
1, except that a Si60Ti20Ni20 silicon-based alloy with a
content of active silicon of about 41.7 atom % was used instead of the
Si68Al8Ni24 silicon-based alloy.

EXAMPLE 3

Manufacture of Negative Electrode

[0095] A negative electrode was manufactured in the same manner as Example
1, except that the diameter of beads employed by the bead mill was about
2 mm, and the revolutions per minute of the bead mill was about 2000 rpm.

[0096] EXAMPLE.4

Manufacture of Negative Electrode

[0097] A negative electrode was manufactured in the same manner as Example
1, except that the composition was prepared using a
Si65Ti17.5Ni17.5 silicon-based alloy with a content of
active silicon of about 52.9 atom % instead of the
Si68Al8Ni24 silicon-based alloy.

EXAMPLE 5

Manufacture of Negative Electrode

[0098] A negative electrode was manufactured in the same manner as Example
1, except that the composition was prepared using a
Si68Ti16Ni16 silicon-based alloy with a content of active
silicon of about 58.8 atom % instead of the Si68Al8Ni24
silicon-based alloy.

COMPARATIVE EXAMPLE 1

Manufacture of Negative Electrode

[0099] A negative electrode was manufactured in the same manner as Example
1, except that the composition was prepared as follows.

[0100] A Si68Al8Ni24 silicon-based alloy as an active
material, Ketjen black as a conductive material, and PAI as a binder were
loaded into a paint shaker at a weight ratio of 88:4:8, followed by 30
minutes of dry mixing. Then, NMP was added thereto and mixed with the
dried product to complete the preparation of the composition.

[0101] The amount of NMP included in the composition was controlled such
that the total solids content of the composition was 48 wt %.

COMPARATIVE EXAMPLE 2

Manufacture of Negative Electrode

[0102] A negative electrode was manufactured in the same manner as Example
1, except that the composition was prepared as follows.

[0103] A Si68Al8Ni24 silicon-based alloy as an active
material, and Ketjen black as a conductive material were loaded into a
Thinky mixer, followed by 30 minutes of dry mixing. Then, NMP and PAI as
a binder were added thereto and mixed with the dried product. Then, the
resultant was mixed using a bead mill for 30 minutes to complete the
preparation of the composition.

[0104] The Si68Al8Ni24 silicon-based alloy as the active
material, Ketjen black as the conductive material, and PAI as the binder
were mixed at a weight ratio of 88:4:8. The diameter of the beads
employed by the bead mill was about 2 mm, and the revolutions per minute
of the bead mill was about 2000 rpm. The amount of NMP included in the
composition was controlled such that the total solids content of the
composition was 48 wt %.

COMPARATIVE EXAMPLE 3

Manufacture of Negative Electrode

[0105] A negative electrode was manufactured in the same manner as Example
1, except that the composition was prepared as follows.

[0106] A Si68Al8Ni24 silicon-based alloy as an active
material, and Ketjen black as a conductive material were loaded into a
paint shaker, followed by 30 minutes of dry mixing. Then, NMP and PAI as
a binder were added thereto and mixed with the dried product. Then, the
resultant was mixed using a bead mill for 30 minutes to complete the
preparation of the composition.

[0107] The Si68Al8Ni24 silicon-based alloy as the active
material, Ketjen black as the conductive material, and PAI as the binder
were mixed at a weight ratio of 88:4:8. The diameter of the beads
employed by the bead mill was about 2 mm, and the revolutions per minute
of the bead mill was about 2000 rpm. The amount of NMP included in the
composition was controlled such that the total solids content of the
composition was 48 wt %.

COMPARATIVE EXAMPLE 4

Manufacture of Negative Electrode

[0108] A negative electrode was manufactured in the same manner as Example
2, except that the composition was prepared as follows.

[0109] The silicon-based alloy as the active material, Ketjen black as the
conductive material, and PAI as the binder were loaded into a paint
shaker at a weight ratio of 88:4:8, followed by 30 minutes of dry mixing.
Then, NMP was added thereto and mixed to complete the preparation of the
composition.

[0110] The amount of NMP included in the composition was controlled such
that the total solids content of the composition was 48 wt %.

COMPARATIVE EXAMPLE 5

Manufacture of Negative Electrode

[0111] A negative electrode was manufactured in the same manner as Example
4, except that the composition was prepared as follows.

[0112] The silicon-based alloy as the active material, Ketjen black as the
conductive material, and PAI as the binder were loaded into a paint
shaker at a weight ratio of 88:4:8, followed by 30 minutes of dry mixing.
Then, NMP was added thereto and mixed to complete the preparation of the
composition.

[0113] The amount of NMP included in the composition was controlled such
that the total solids content of the composition was 48 wt %.

COMPARATIVE EXAMPLE 6

Manufacture of Negative Electrode

[0114] A negative electrode was manufactured in the same manner as Example
5, except that the composition was prepared as follows.

[0115] The silicon-based alloy as the active material, Ketjen black as the
conductive material, and PAI as the binder were loaded into a Thinky
mixer at a weight ratio of 88:4:8, followed by 30 minutes of dry mixing.
Then, NMP was added thereto and mixed to complete the preparation of the
composition.

[0116] The amount of NMP included in the composition was controlled such
that the total solids content of the composition was 48 wt %.

MANUFACTURE EXAMPLE 1: Manufacture of Coin Cell

[0117] A CR-2016 standard coin cell was manufactured using the negative
electrode prepared according to Example 1, lithium metal as a reference
electrode, a polypropylene separator (Cellgard 3510), and an electrolyte
in which 1.3M LiPF6 was dissolved in a mixture including ethylene
carbonate (EC) and diethyl carbonate (DEC)(at a weight ratio of 3:7).

MANUFACTURE EXAMPLE 2

Manufacture of Coin Cells

[0118] A coin cell was manufactured in the same manner as Manufacture
Example 1, except that the negative electrode prepared according to
Example 2 was used instead of the negative electrode prepared according
to Example 1.

MANUFACTURE EXAMPLE 3

Manufacture of Coin Cells

[0119] A coin cell was manufactured in the same manner as Manufacture
Example 1, except that the negative electrode prepared according to
Example 3 was used instead of the negative electrode prepared according
to Example 1.

MANUFACTURE EXAMPLE 4

Manufacture of Coin Cells

[0120] A coin cell was manufactured in the same manner as Manufacture
Example 1, except that the negative electrode prepared according to
Example 4 was used instead of the negative electrode prepared according
to Example 1.

MANUFACTURE EXAMPLE 5

Manufacture of Coin Cells

[0121] A coin cell was manufactured in the same manner as Manufacture
Example 1, except that the negative electrode prepared according to
Example 5 was used instead of the negative electrode prepared according
to Example 1.

COMPARATIVE MANUFACTURE EXAMPLE 1

Manufacture of Coin Cells

[0122] A coin cell was manufactured in the same manner as Manufacture
Example 1, except that the negative electrode prepared according to
Comparative Example 1 was used instead of the negative electrode prepared
according to Example 1.

COMPARATIVE MANUFACTURE EXAMPLE 2

Manufacture of Coin Cells

[0123] A coin cell was manufactured in the same manner as Manufacture
Example 1, except that the negative electrode prepared according to
Comparative Example 2 was used instead of the negative electrode prepared
according to Example 1.

COMPARATIVE MANUFACTURE EXAMPLE 3

Manufacture of Coin Cells

[0124] A coin cell was manufactured in the same manner as Manufacture
Example 1, except that the negative electrode prepared according to
Comparative Example 3 was used instead of the negative electrode prepared
according to Example 1.

COMPARATIVE MANUFACTURE EXAMPLE 4

Manufacture of Coin Cells

[0125] A coin cell was manufactured in the same manner as Manufacture
Example 1, except that the negative electrode prepared according to
Comparative Example 4 was used instead of the negative electrode prepared
according to Example 1.

COMPARATIVE MANUFACTURE EXAMPLE 5

Manufacture of Coin Cells

[0126] A coin cell was manufactured in the same manner as Manufacture
Example 1, except that the negative electrode prepared according to
Comparative Example 5 was used instead of the negative electrode prepared
according to Example 1.

COMPARATIVE MANUFACTURE EXAMPLE 6

Manufacture of Coin Cells

[0127] A coin cell was manufactured in the same manner as Manufacture
Example 1, except that the negative electrode prepared according to
Comparative Example 6 was used instead of the negative electrode prepared
according to Example 1.

EVALUATION EXAMPLE 1

Scanning Electron Microscope Analysis

[0128] The negative electrodes manufactured according to Example 1 and
Comparative Example 2 were analyzed using an scanning electron microscope
(SEM), and the results thereof are shown in FIGS. 3 and 4. The negative
electrodes analyzed by the SEM were not yet pressed following the coating
and drying of the compositions on the current collectors.

[0129] Referring to FIG. 4, it was confirmed that the active material in
the electrode of Comparative Example 2 was pulverized into too small
particles. On the other hand, referring to FIG. 3, it was confirmed that
the active material in the electrode of Example 1 was not pulverized into
too small particles due to the wet mixing and milling process using the
bead mill.

EVALUATION EXAMPLE 2

Surface Roughness of Negative Electrode

[0130] Surface roughness and roughness deviation of the surfaces of the
negative electrodes of Example 1-5 and Comparative Examples 1-6 are shown
in Table 1 below.

[0131] The surface roughness of the negative electrode is the surface
roughness after pressing. The surface roughness was measured as follows.

[0132] The negative electrodes of Example 1-5 and Comparative Examples 1-6
were cut to a sample size, and the samples were scanned (scan interval:
about 2.5 mm, and scan range: about 25 mm) 9 times using a non-contact
laser surface analyzer (usurf custom manufactured by NanoFocus AG
Company) to obtain surface roughness mean values and roughness deviation
values. The surface roughness mean values and the roughness deviation
mean values were used as the surface roughness values and the roughness
deviation values, respectively.

[0133] Referring to Table 1, it was confirmed that the negative electrodes
of Examples 1-5 had lower and more uniform surface roughness than the
negative electrodes of Comparative Examples 1-6.

EVALUATION EXAMPLE 3

Mixed Density and Current Density of Negative Electrode

[0134] The mixed density and current density of the negative electrodes of
Example 1 and Comparative Examples 1-4 were measured, and the results
thereof are shown in Table 2 below.

Mixed Density

[0135] Mixed density was measured by dividing the weight of the negative
electrode components other than the current collector (i.e., active
material, conductive material, binder, etc.) by the volume of the
electrode.

[0136] Referring to Table 2, it was confirmed that the negative electrode
of Example 1 had a higher current density than the negative electrodes of
Comparative Examples 1-4. Evaluation Example 4. Volumetric capacity and
expansion ratio of negative electrode

[0137] The volumetric capacity, expansion ratio, and volumetric capacity
in consideration of expansion of the negative electrodes of Example 1 and
Comparative Examples 1-4 were measured, and the results thereof are shown
in FIG. 5 and Table 3 below. The volumetric capacity, the expansion
ratio, and the volumetric capacity in consideration of expansion were
evaluated as follows.

(1) Volumetric Capacity

[0138] Volumetric capacity was measured according to Equation 1 below.

[0139] The coin cells manufactured according to Manufacture Example 1 and
Comparative Manufacture Examples 1-4 (including the negative electrodes
of Example 1 and Comparative Examples 1-4) were charged at a rate of 0.1
C, and then the coin cells were disassembled in a dry room. The increase
in thickness of each of the negative electrodes was measured, and the
expansion ratios of the negative electrodes were calculated according to
Equation 2 below.

[0141] Referring to Table 3 and FIG. 5, it was confirmed that the negative
electrode of Example 1 had a lower expansion ratio and a higher
volumetric capacity in consideration of expansion than the negative
electrodes of Comparative Examples 1-4.

EVALUATION EXAMPLE 5

Charging and Discharging Test

[0142] The initial charge efficiency (I.C.E) and discharge capacity of the
coin cells manufactured according to Manufacture Example 1 and
Comparative Manufacture Examples 1-4 were measured, and the results
thereof are shown in Table 4 below.

[0143] First, the coin cells of Manufacture Example 1 and Comparative
Manufacture Examples 1-4 were charged and discharged once at 0.1 C to
perform a formation process. Thereafter, charging and discharging was
performed once at 0.2 C to identify initial charging and discharging
characteristics. Then, at 1 C, charging and discharging were repeatedly
performed 100 times to identify cycle characteristics. Charging was set
such that charging began in a constant current (CC) mode, and then the
mode was changed into a constant voltage (CV) mode, and cut-off occurred
at 0.01 C. Discharging was set such that cut-off occurred at 1.5V in the
CC mode.

(1) Initial charging and discharging efficiency

[0144] The initial charging and discharging efficiency was calculated
according to Equation 4 below.

[0146] Referring to Table 4, it was confirmed that the lithium secondary
battery of Manufacture Example 1 had improved I.C.E. and discharge
capacity compared to the lithium secondary batteries of Comparative
Manufacture Examples 1-4.

EVALUATION EXAMPLE 6

Cycle Lifespan

[0147] First, the coin cells of Manufacture Example 1 and Comparative
Manufacture Examples 1, 2, 3, and 5 were charged and discharged once at
0.1 C to perform a formation process. Thereafter, charging and
discharging was performed once at 0.2 C to identify initial charging and
discharging characteristics. Then, at 1 C, charging and discharging were
repeatedly performed 100 times to identify cycle characteristics.
Charging was set such that charging began in a constant current (CC) mode
and then the mode was changed into a constant voltage (CV) mode, and
cut-off occurred at 0.01 C. Discharging was set such that cut-off
occurred at 1.5V in the CC mode.

[0148] The discharge capacity change with respect to cycle was evaluated,
and the evaluation results are shown in FIG. 6.

[0149] Referring to FIG. 6, it was confirmed that the lithium secondary
battery of Manufacture Example 1 had better cycle lifespan
characteristics than the lithium secondary batteries of Comparative
Manufacture Examples 1, 2, 3 and 5.

EVALUATION EXAMPLE 7

Lifespan Characteristics

[0150] First, the coin cells of Manufacture Examples 2, 4, 5 and
Comparative Manufacture Examples 2, 5 and 6 were charged and discharged
once at 0.1 C to perform a formation process. Thereafter, charging and
discharging was performed once at 0.2 C to identify initial charging and
discharging characteristics. Then, at 1 C, charging and discharging were
repeatedly performed 100 times to identify cycle characteristics.
Charging was set such that charging began in a constant current (CC)
mode, and then the mode was changed into a constant voltage (CV) mode,
and cut-off occurred at 0.01 C. Discharging was set such that cut-off
occurred at 1.5V in the CC mode.

[0151] The capacity retention rate in the 100th cycle is represented
by Equation 5 below, and the calculation results are shown in Table 5.

[0152] Referring to Table 5, it was confirmed that the coin cells of
Manufacture Examples 2, 4 and 5 had higher capacity retention rates than
the coin cells of Comparative Manufacture Examples 2, 5, and 6.

[0153] While the certain exemplary embodiments of the present invention
have been illustrated and described, those of ordinary skill in the art
would recognize that various modifications and changes can be made to the
described embodiments without departing from the spirit and scope of the
present invention, as defined in the appended claims.

Patent applications by Chang-Ui Jeong, Yongin-Si KR

Patent applications by Chun-Gyoo Lee, Yongin-Si KR

Patent applications by Jae-Hyuk Kim, Yongin-Si KR

Patent applications by Jong-Seo Choi, Yongin-Si KR

Patent applications by Seung-Uk Kwon, Yongin-Si KR

Patent applications by Soon-Sung Suh, Yongin-Si KR

Patent applications by Sung-Hwan Moon, Yongin-Si KR

Patent applications by Yo-Han Park, Yongin-Si KR

Patent applications by Yury Matulevich, Yongin-Si KR

Patent applications in class Iron component is active material

Patent applications in all subclasses Iron component is active material